The invention relates to a process for the isomerization of alkenes having a terminal double bond to give alkenes having an internal double bond in the presence of hydrogen on a cation exchanger in the H.sup.+ form which is coated with a hydrogenation-active metal.
Linear alkenes, including those in which a possible branching does not originate at a C atom forming the double bond, have in the past proved valuable raw materials for various reactions. Thus, alkenes having a terminal double bon are valued as monomers or comonomers for the preparation of polyolefins having valuable properties on the other hand, alkenes having an internal double bond are in demand as alkylating agents for the alkylation of n-alkanes and/or isoalkanes, which gives valuable motor fuels ("alkylate petrol"). In these alkylation reactions, alkenes having an internal double bond are preferred to those having a terminal double bond, because their alkylates have better properties in the fuel field. Thus, for example, n-but-1-ene with n-butane/isobutane gives an alkylate petrol having a Reseach Octane Number RON of 91, whereas n-but-2-ene under the same conditions gives an alkylate having an RON of 97.
The hydrocarbon mixtures formed in cracking plants (for example steam crackers of FCC=fluid catalyst crackers) are, above all, a suitable source of alkenes of the type mentioned. The alkenes having a terminal or internal double bond of the same number of C atoms which are present in such mixtures from crackers are in a thermodynamic equilibrium which depends on the properties of the particular pair of alkenes. At high cracker temperatures, this thermodynamic equilibrium is more strongly in the side of the alkene having the terminal double bond, whereas at lower temperatures it is shifted more and more towards the alkenes having an internal double bond.
There has, therefore, been no lack of attempt to isomerize catalytically alkenes having a terminal double bond into alkenes having an internal double bond and to bring the ratio of the one to the other nearer to the thermodynamic equilibrium at a temperature lower than the cracker temperature. An isomerization of this type is possible, for example, on a palladium catalyst, varying statements having been made in the literature concerning necessary auxiliaries. Thus it is reported in FR No. 7,828,723 that a little carbon monoxide must be added for isomerization in the presence of hydrogen. Furthermore, the supports for palladium catalysts of this type are high-purity, and therefore inert, mineral substances. In DE-OS (German Published Specification) No. 3,140,573, for example, high-purity Al.sub.2 O.sub.3 containing 0.3% by weight of Pd is mentioned, the heat of neutralization of the catalyst, measured as the heat of neutralization when ammonia is absorbed, having the extremely low value of 6 cal/g.
The DE-OS (German Published Specification) mentioned describes, in addition, a two-stage upgrading process for a C.sub.4 -olefin fraction, in which process the isobutene present in the olefin fraction is oligomerized in a first stage on a fluorinated aluminium oxide, boron/aluminium oxide or silicon dioxide/aluminium oxide, and in which, in the second stage, n-but-1-ene is partly isomerized to give n-but-2-ene in the presence of hydrogen in a separate catalyst bed on the Pd catalyst described above.